Environment - Global Climate Change

Global Climate Change

Climate Change Facts
September 9, 1997

Issue 1 U.S. House of Representatives

Minority Staff, Committee on Government Reform and Oversight

What is the Greenhouse Effect?
Rapid changes in the earth's climate are one of the most significant environmental concerns of the 21st century. The prospect of changes in climate induced by human activity is a well-understood phenomenon, and there is now a growing scientific consensus that human activities threaten to increase heat-trapping gases in the atmosphere that can rapidly raise the earth's average temperature to dangerous levels.

This is the first in a series of fact sheets on climate change. It describes the "greenhouse effect" caused by carbon dioxide and other gases in the atmosphere.

The Solar Radiation Balance. The understanding of climate change begins with the natural balance of energy between the earth and the sun. All of the energy that drives life on earth begins with the sun. Energy is emitted by the sun in the form of visible and ultraviolet light (see diagram). Roughly one-third of this energy is reflected back through the atmosphere, because it hits glaciers, snow, and other light-colored surfaces that reflect light. The rest of the solar radiation is absorbed by oceans, land surfaces, plant and animal life, and the atmosphere itself.

The short wavelength radiation from the sun is converted to heat when it is absorbed by matter on the earth's surface. What we know as heat is a form of energy with wavelengths too long to be visible and is technically called "infrared radiation." In nature, heat will disperse until an equilibrium in temperature is reached. For example, when a loaf of bread emerges from an oven it emits heat until it gradually cools to room temperature. In the same process, the energy absorbed by the earth from the sun is balanced over time by the re-radiation of heat away from the earth.

The Natural Greenhouse Effect. What keeps the earth warm enough to sustain life are naturally-occurring gases such as carbon dioxide (CO2), nitrous oxide (NOX), methane (CH4), ozone (O3), and water vapor that collect in the upper atmosphere. These gases have molecular configurations that trap infrared radiation within the atmosphere, and re-emit this energy in all directions. This trapping of the heat radiated from the earth holds in energy that would normally be lost from the atmosphere and thus causes a net increase in the earth's temperature. This temperature increase is called "radiative forcing," and can be predicted with precision based on the concentrations of these gases in the earth's atmosphere. Without the natural greenhouse effect of the atmosphere, most of the heat from the sun would be radiated away from the earth and lost to space, and the average temperature of the earth would be about 0 degrees Fahrenheit. Thanks to radiative forcing, the average temperature of the earth is currently about 60 degrees Fahrenheit.

This process is called the "greenhouse effect" because the gases act like the walls of a greenhouse, trapping heat from the sun which would ordinarily be lost to the atmosphere. Just as a greenhouse is able to sustain tropical plants in chilly North American winters, "greenhouse gases" sustain life on earth.

History of Understanding the Greenhouse Effect. The first speculations about the relationship between gases in the atmosphere and climate emerged over a century ago. In the 1860's, English physicist J. Tyndall proposed that variations in the composition of the earth's atmosphere could alter climate. At the turn of the century, Swedish chemist Svante Arrhenious predicted that CO2 emissions from coal combustion would raise the temperature of the earth. Around the same time, T.C. Chamberlain, an American geologist, suggested that large climatic variations such as periods of glaciation could be attributable to the concentration of CO2 in the atmosphere.

The relationship between the chemistry of the earth's atmosphere, industrial activity, and global climate has been extensively studied in the years since, and in 1988 an international panel of peer-reviewed scientists (the Intergovernmental Panel on Climate Change or IPCC) was commissioned by the World Meteorological Organization and the United Nations to gather and assess scientific understanding of climate change and to formulate response strategies. The IPCC is now regarded as an objective source on the science of climate change.

Human Amplification of the Greenhouse Effect. The greenhouse effect is a policy concern today because there is scientific consensus that human activities are significantly increasing the concentration of greenhouse gases in the atmosphere. The major sources of these emissions in the U.S. include burning fossil fuels, the use of fertilizers and other agricultural practices, and changes in land-use. In developing countries, the practice of burning forested land for grazing is also a major source of CO2 . Since the Industrial Revolution in the mid-18th Century, the concentrations of carbon dioxide have increased by over 30%, methane by at least 145%, and nitrous oxide by over 15%, bringing them to their highest levels in 220,000 years.

As will be discussed in future fact sheets, many scientists now believe that these rapid increases in greenhouse gases have already caused an increase in the earth's average temperature and are likely to cause even greater increases in the future.